Given the low levels of engagement in physical activity among young adults, this mixed methods study was designed to increase our understanding of the factors that influence their participation. Interest motivation theory provided the framework for examining the multi-dimensionality of situational interest as a component that is predictive of engagement in physical activity. College students enrolled in tennis classes at a large research university were recruited to participate in the research. A survey assessing the sources of situational interest was administered to 82 students, and eight students were interviewed so that we could obtain a deeper understanding of the quantitative results. A stepwise multiple regression model revealed four sources (enjoyment, novelty, exploration, and attention demand) that operated as predictors of total situational interest (r[superscript 2] = 0.93). From the interpretive analysis of the interview data, three themes emerged (autonomy, competence, and social support). Given these findings, recommendations for modifying physical activity classes are provided.